Conventionally, an optical tomographic image acquiring device using OCT (Optical Coherence Tomography) measurement is used in some cases to acquire an optical tomographic image of living tissues. The optical tomographic image acquiring device divides low-coherence light emitted from a light source into measurement light and reference light and then multiplexes reflected light, which is from a measurement target when the measurement light is directed to the measurement target, or backscattered light and the reference light to acquire an optical tomographic image based on the intensity of interference light of the reflected light and the reference light (Patent Literature 1). Hereinafter, the reflected light from the measurement target and the backscattered light will be collectively described as reflected light.
There are roughly two types of OCT measurement, TD-OCT (Time domain OCT) measurement and FD-OCT (Fourier Domain OCT) measurement. The TD-OCT measurement is a method of measuring the interference light intensity while changing the optical path length of the reference light to acquire a reflected light intensity distribution corresponding to the position in a depth direction (hereinafter, called “depth position”) of the measurement target.
Meanwhile, the FD-OCT measurement is a method of measuring the interference light intensity of each spectral component of light without changing the optical path length of the reference light and the signal light, and a computer applies a frequency analysis represented by a Fourier transform to a spectral interference intensity signal obtained here to acquire the reflected light intensity distribution corresponding to the depth position. The PD-OCT measurement is recently drawing attention as a method that allows high-speed measurement, because mechanical scanning that exists in the TD-OCT is not necessary.
Typical examples of device configurations for performing the FD-OCT measurement include two types, an SD-OCT (Spectral Domain OCT) device and an SS-OCT (Swept Source OCT). The SD-OCT device uses wideband low-coherence light, such as an SLD (Super Luminescence Diode), an ASE (Amplified Spontaneous Emission) light source, and white light, as a light source, uses a Michelson interferometer or the like to divide the wideband low-coherence light into measurement light and reference light, directs the measurement light to a measurement target, interferes reflected light returned at that time and the reference light, uses a spectrometer to dissolve the interference light into frequency components, uses a detector array including elements such as photodiodes arranged in an array to measure the interference light intensity of each frequency component, and applies a Fourier transform to an obtained spectral interference intensity signal by a computer to thereby form an optical tomographic image.
Meanwhile, the SS-OCT device uses a laser that temporally sweeps the optical frequency as a light source, interferes reflected light and reference light at each wavelength, measures the time waveform of a signal corresponding to the time change of the optical frequency, and applies a Fourier transform to an obtained spectral interference intensity signal by a computer to thereby form an optical tomographic image.
Although the OCT measurement is a method for acquiring an optical tomographic image of a specific area as described above, an endoscope can determine the extent of the invasion of a cancer lesion by, for example, detecting the cancer lesion through observation by a normal illumination light endoscope or a special light endoscope and applying an OCT measurement to the area. The optical axis of the measurement light can be two-dimensionally scanned to acquire three-dimensional information along with depth information based on the OCT measurement.
Integration of the OCT measurement and a three-dimensional computer graphic technique allows displaying a three-dimensional structure model with micrometer-order resolving power. Therefore, the three-dimensional structure model based on the OCT measurement will be called an optical three-dimensional structure image.
For example, the cancer invasion depth of esophagus is observed by the OCT. An OCT image of esophagus depicts, from the near side, a thin epithelial layer and a strongly scattered basement membrane, relatively strongly scattered lamina propria mucosae below the epithelial layer and the basement membrane, and relatively weakly scattered muscularis mucosae, a strongly scattered submucosal layer, as well as a weakly scattered muscular layer below the lamina propria mucosae.
An example of a tissue structure change caused by the development of cancer will be described. Epithelium hypertrophy is developed when the cancer develops and grows on the epithelial layer. It is known that at this period, new blood vessels extend from blood vessels in a submucosal layer to the mucosal layer, toward the cancer, and the new blood vessels are formed around the cancer cells beyond the basement membrane. When the cancer progresses, the cancer breaks the basement membrane to invade the lamina propria, and if the cancer further progresses, the invasion depth increases toward the muscularis mucosae, the submucosal layer, and the muscular layer.
The cancer that has not invaded the basement membrane is called an “intraepithelial neoplasm”, which serves as an indication that the cancer will be cured if removed. It is important to determine whether the cancer has invaded below the basement membrane to detect an early-stage cancer earlier for a minimally invasive treatment of the cancer before there is a risk of spreading. If the cancer has invaded beyond the basement membrane, whether the cancer has invaded beyond the muscularis mucosae is important as the next indication. The possibility of metastasis is low if the cancer is not beyond the muscularis mucosae, and an endoscopic ablative therapy is selected. On the other hand, the possibility of metastasis is high if the cancer is beyond the muscularis mucosae, and an open-chest surgery or a radiation therapy is selected. It is important to determine whether the cancer has invaded below the muscularis mucosae for a minimally invasive treatment of an early-stage cancer. Therefore, it is expected to extract and image only a specific membrane or layer, such as a basement membrane or muscularis mucosae. However, there is no method of directly observing the state of the basement membrane.
A method of extracting a specific scattering intensity of, for example, ocular fundus by the OCT to extract a layer structure is disclosed (Patent Literature 2). To extract the layer structure, a one-dimensional differential filter or the like in a depth direction is specifically used to extract the layer structure or the boundary of the layer. The layer structure of the ocular fundus is clear, and there is a little change in the structure. Therefore, there is not much error in the extraction based on the method. However, there is no example of the implementation of the method in digestive tracts, such as esophagus.
It is known that if cancer develops on the epithelial layer, new blood vessels are formed on the mucosal layer toward the cancer. In the case of early-stage cancer of esophagus, the new blood vessels pass through the submucosal layer and the basement membrane to extend to the mucosal epithelial layer to form an IPCL (intra-epithelial papillary capillary loop). If the cancer progresses, the cancer breaks the basement membrane and enters the submucosal layer. The new blood vessels are formed in random directions toward the cancer. In normal endoscopy, a method of determining the grade of cancer from the density distribution and the shapes of new blood vessels that can be seen through from the surface is implemented.